AQA Module C2 Reversible Reactions & The Haber Process

Most chemical reactions only ‘go’ one way. A few reactions go both ways. These are called ‘REVERSIBLE’ Reactants Products

HAND WARMERS USE A REVERSIBLE REACTION These are plastic bags containing sodium acetate crystals in solution. 1.Put bag in hot water for a few minutes and the cloudy liquid goes clear as the crystals dissolve 2.Let it cool down 3.Click the metal disc inside and the clear liquid goes cloudy again and gets HOT. FORWARD REACTION REVERSE REACTION Click disc HEAT IN ENDOTHERMIC HEAT OUT EXOTHERMI C

In a reversible reaction, the REACTANTS turn into PRODUCTS AND the PRODUCTS can be turned back into the REACTANTS BLUE COPPER WHITE COPPER + WATER SULPHATE SULPHATE (STEAM) REACTANTPRODUCTS FORWARD REACTION REVERSE REACTION Heat IN Heat OUT ENDO- THERMIC EXO- THERMIC

White copper sulphate (PRODUCT) Blue copper sulphate (REACTANT) Water (PRODUCT) Equilibrium If we make a CLOSED SYSTEM by preventing any reactants or products escaping…. The RATE of the FORWARD REACTION balances the RATE of the REVERSE REACTION So we have both REACTANTS AND PRODUCTS CONSTANTLY TURNING INTO EACH OTHER And the system is in EQUILIBRIUM

Heat IN Removal of Product (s) If we want to make and collect one (or both) of the products in a reversible reaction, it must be steadily removed from the system as we go WATER REMOVED so REVERSE REACTION can’t happen so PRODUCT can be collected

The production of ammonia is a very important process as it is used to make fertilizers to grow more food crops. Ammonia is made from: nitrogen, which is removed from the air hydrogen, made from methane (natural gas) The HABER PROCESS for producing AMMONIA The Haber Process is a reversible reaction where both reactants and the product are GASES. NN HH N H H H

H NN HHHHHH NN HHHHH N H H H THE REACTION Production of ammonia from nitrogen and hydrogen N 2 + 3H 2 2NH 3 HH HH N N HH HH HH HH N H H H Nitrogen (g) + hydrogen (g) ammonia (g)

N HHH N HHHH NN HHHHH FORWARD REACTION: Hydrogen & Nitrogen make Ammonia REVERSE REACTION: Ammonia makes Hydrogen & Nitrogen EXOTHERMIC ENDOTHERMIC BOTH REACTIONS ARE HAPPENING AT ONCE

HYDROGEN NITROGEN AMMONIA HYDROGEN & NITROGEN IN UNUSED HYDROGEN & NITROGEN RECYCLED hydrogen + nitrogen ammonia TEMPERATURE and PRESSURE of the reaction vessel can be controlled Mixture cooled here. AMMONIA condenses LIQUID AMMONIA REMOVED % YIELD = % AMMONIA in main reaction vessel

How to make the most ammonia quickly & cheaply ? TEMPERATURE PRESSURE We can change 2 conditions inside the reaction vessel: 500°C 20°C (normal temp) 500 atmospheres 1 atmosphere (normal pressure) ££ CHEAP ££ ££ EXPENSIVE ££

EFFECT OF TEMPERATURE HYDROGEN NITROGEN AMMONIA Because the forward reaction is exothermic (’releases heat’), the % YIELD of ammonia is GREATER at LOWER TEMPERATURES (The ammonia molecules tend to split up again at high temps) COOLHOT So it would seem that the temperature needs to be LOW…… BUT … LOW TEMPERATURES make the rate of reaction SLOW so you would have to wait a long time… So a higher temperature (450°C) is actually used to make the ammonia FASTER even though the yield is lower.

EFFECT OF PRESSURE HH HH N N HH HH HH HH N H H H N H H H 4 molecules 2 molecules HYDROGEN NITROGEN AMMONIA Because the FORWARD reaction produces a SMALLER NUMBER OF MOLECULES, a HIGHER PRESSURE makes a LARGER YIELD of AMMONIA BUT using a HIGH PRESSURE means much stronger and MORE EXPENSIVE pipes and reaction vessels are needed. So.. a MEDIUM PRESSURE of 200 atmospheres is used.

% 40% 60% 80% 100% 0% 200°C 300°C 400°C 500°C Pressure (atmospheres) Yield of ammonia A COMPROMISE solution 450°C and 200 atmos. TEMP:LOW enough for a reasonable yield but HIGH enough for a fast reaction PRESSURE: LOW enough to not need expensive reinforced apparatus but HIGH enough to give a reasonable yield 450°C IRON CATALYST used to speed reaction up further Gives about 30% yield